Typically, someone who suffers from hearing loss wears a hearing assistive device, such as a hearing aid. Hearing aids are electro acoustical devices worn inside the ear to compensate for a hearing impairment by amplifying the local sound field. Generally, hearing aids operate in either a microphone mode or a telecoil mode. In the microphone mode, sound waves incident upon a microphone that is integrated in the hearing aid are converted to an electrical audio signal. In the telecoil mode, an induction coil (also referred to as a telecoil or T-coil) which may also be inside the hearing aid picks up the local magnetic field that has been modulated by the receiver (earpiece speaker) of a telephone handset. In either mode, the resultant electrical audio signal that has been picked up is subsequently processed, amplified and converted to sound (by a small speaker inside the hearing aid) that can be heard by the user.
Hearing aids do not always function well with some portable audio devices, such as mobile phones. One problem experienced when using a hearing aid in conjunction with a mobile phone is that the microphone inside the hearing aid may pick up unwanted ambient acoustic noise from the surrounding background environment, in addition to the desired speech coming from the mobile phone earpiece speaker (receiver), which makes it difficult for the user to discern the desired speech. However, when a hearing aid is switched to the T-coil mode, the hearing aid microphone may be deactivated, and the T-coil inductively couples the output audio signal (from a speaker in the mobile phone) to the hearing aid. As such, environmental or background acoustic noise is not amplified by the hearing aid when the T-coil is being used as a pick-up.
Hearing aid compatible (HAC) mobile phones are becoming more commonly available to the public. In addition to the typical acoustic receiver, HAC phones may also include a separate loop of wire (referred to as a telecoil or T-coil) for inductive coupling with the T-coil of a nearby hearing aid. Such phones are thus compatible with both the microphone of a hearing aid and its T-coil. These mobile phones traditionally include a selector switch that enables a user to manually select a HAC mode of operation. In that mode of operation, the audio processing applied to an audio signal may be modified to change the phone's audio frequency response so as to better accommodate the microphone of a hearing aid. Another change that may be made when the HAC mode has been selected is to allow the processed audio signal to drive a telecoil inside the mobile phone.
However, a user may find having to manually select the mode of operation of the mobile phone inconvenient and time consuming. For example, a user without a hearing impairment may wish to hand the mobile phone over to a person who is wearing a hearing aid, during an on-going call for instance. In this case, the user would need to manually select the HAC mode of operation before handing the phone over to the person wearing the hearing aid. Accordingly, automatic techniques for detecting the presence of a nearby hearing aid have been suggested.